Evaluation of high strain response in lead-free BNBTFS-xNb ceramics by structure and ferroelectric characterizations

Lead-free piezoelectric ceramics of the form (Bi_0.5Na_0.5)_0.945Ba_0.055 [Ti_0.98(Fe_0.5Sb_0.5)_0.02]_(1-x)Nb_xO₃ (BNBTFS-xNb with x = 0.00–0.03) in the vicinity of the morphotropic phase boundary were synthesized by a conventional mixed oxide route. The crystal structure, microstructure, and electromechanical properties were investigated as a function of different Nb⁵⁺ concentrations. Increasing amount of Nb⁵⁺ disrupts the long-range ferroelectric order and induces a relaxor phase with pseudocubic symmetry, which results in a large strain (S_max = 0.39%, equivalent to 666 p.m./V) for a critical composition (x = 0.01). Furthermore, ex situ X-ray diffraction of the poled and unpoled ceramics (x = 0.00 and 0.01) was performed, focusing on the origin of the high strain response. The results suggest that such high strain response originates from the compositionally induced phase transition and electric field–induced reversible phase transition from the ergodic relaxor phase to a temporary long-range metastable (non-ergodic) ferroelectric phase.